Caption: Habitable alien planet. Computer artwork of an Earth-like alien planet and its sun. Its moon is at lower left. A planet's orbit and its size have to fit within a narrow range in order to have a surface temperature suitable for liquid water, and to retain an atmosphere. The number of other habitable planets in the Milky Way is not known, but has been estimated at anything between 1 (just Earth) and as many as 10 billion.